1. Field of the Invention
The invention relates generally to drop detection in a medical liquid drop chamber and, more specifically, concerns a drop detection method and apparatus for use in an ambulatory or household environment.
2. Description of Related Art
Medical drop chambers are used in various medical devices for metering and monitoring the flow rate of a fluid being administered to a patient. In a given drop chamber, each drop has a uniform volume of fluid. Therefore, by counting the number of drops falling in a given time period, the flow rate can be calculated easily. Such drop chambers are used, for example, in gravity-driven or pump-driven infusion systems.
Devices are known in the art for automatically sensing the drops in a chamber. These may, for example, be connected to circuits that can compute and display the flow rate or to alarms that indicate when the flow rate is too high or too low. These drop detectors are often optical sensors that react to a drop breaking optical communication between a light source and a sensor. In a controlled environment, such as a hospital, few outside conditions affect the optical sensors. The ambient light is fairly uniform throughout the environment and the drop chamber is relatively immobile and usually kept upright.
However, in either an ambulatory or household environment, several factors that may affect the optical sensors must be handled properly by the drop sensor to avoid false readings or alarms. These factors include widely varying ambient light conditions and excessive movement and tilting of the drop chamber, especially in ambulatory situations. False readings caused by these factors are a major reason for physicians' reluctance to use the ambulatory devices. It has therefore been a goal in the art that the drop detectors be capable of increased sensitivity to the drops, while being immune to the ambient light variation or movement and change in orientation of the chamber.
U.S. Pat. No. 4,720,636 to Benner, Jr. discloses a drop detection structure and detection circuitry that includes two photodetectors, one for sensing a decrease in light caused by a drop passing in front of it, and another for detecting an increase in light caused when a drop passes nearby and reflects additional light. A drop would pass nearby, for example, if the chamber were tilted. However, in the event of a very high tilt angle, coherent drops are not always formed. The liquid may enter the chamber and immediately spread onto the interior surface of the chamber, rather than falling to the bottom of the chamber.
U.S. Pat. No. 4,718,896 to Arndt et al. discloses a drop detector that includes an array of light emitter/sensor pairs arranged to detect drops falling at angles of up to 30 degrees from the normal, vertical orientation. Tilt angles greater than 30 degrees are found in everyday use of the medical devices containing these detectors, rendering the detectors of this patent only partially effective.
In accordance with the invention, a drop detector system and circuit is provided. The system includes an infrared light emitter and detector system for detecting drops passing by its optical sensing path. The drop detector circuit senses the passing of a drop through the optical sensing path and ameliorates the effect of changing ambient light on the circuit's ability to detect the falling drops. The drop detector circuit includes a DC signal blocking element, preferably a capacitor, electrically interposed between the photodiode and amplifiers to block amplification of signals caused by ambient light. After amplification, the signals are passed through a low pass filter and a differentiator circuit to further block signals caused by undesirable factors.